Cobalt(II) bromide

Properties

When anhydrous, cobalt(II) bromide appears as green crystals. The hexahydrate loses four waters of crystallization molecules at 100 °C forming the dihydrate:

CoBr₂.6H₂O → CoBr₂.2H₂O + 4 H₂O

Further heating to 130 °C produces the anhydrous form:

CoBr₂.2H₂O → CoBr₂ + 2 H₂O

The anhydrous form melts at 678 °C. At higher temperatures, cobalt(II) bromide reacts with oxygen, forming cobalt(II,III) oxide and bromine vapor.

Preparation

Cobalt(II) bromide can be prepared as a hydrate by the reaction of cobalt hydroxide with hydrobromic acid:

Co(OH)₂(s) + 2HBr(aq) → CoBr₂.6H₂O(aq)

Anhydrous cobalt(II) bromide may be prepared through the direct reaction of elemental cobalt and liquid bromine.

Reactions and uses

The classical coordination compound bromopentaamminecobalt(III) bromide is prepared by oxidation of a solution of cobalt(II) bromide in aqueous ammonia.

2 CoBr₂ + 8 NH₃ + 2 NH₄Br + H₂O₂ → 2 [Co(NH₃)₅Br]Br₂ + 2 H₂O

Triphenylphosphine complexes of cobalt(II) bromide have been used as a catalysts in organic synthesis.

Safety

Exposure to large amounts of cobalt(II) can cause cobalt poisoning. Bromide is also mildly toxic.